Does the code:

if(live_call()){ return live_result; }

need to be on every Event (ex. Create, Step, etc.) of an object? Or can it be placed in the Create event and still affect the other events in the same object?

It does have to be in each event, which is because each event is separate in terms of execution.

If you are using GMS2/GMS2.3, you can make a macro for that entire line for easier entry.